Method and system for measuring position on surface capacitance touch panel using a flying capacitor

ABSTRACT

A touch panel having a substantially even coating of a conductive material on a non-conductive substrate and then covering the conductive material with a dielectric material, wherein a novel current measuring circuit reduces the effect of stray capacitance on the accuracy of a current measurement so that the relative X and Y position of an object on the touch panel can be determined using simple ratio equations.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims priority to, and incorporates by reference all ofthe subject matter included in the provisional patent application docketnumber 4455.CIRQ.PR, having Ser. No. 61/116,592 and filed on Nov. 20,2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to touchpad technology. Morespecifically, the present invention is a new method of determining theposition of a pointing object on a surface capacitance touch panel.

2. Description of Related Art

A well-known touchpad technology uses a surface capacitance touch panel10 as shown in FIG. 1. Such a touch panel 10 is a solid sheet of aconductive material 16 disposed on an insulating substrate 18 such asglass, with sensors 1.2 disposed at the corners. The traditional methodof measuring the position of a pointing object 14 or the “touchposition” on the surface capacitance touch panel 10 is to apply an ACsignal on all four corners of the touch panel's conductive layer 16. Theconductive layer 16 can be made, for example, of Indium Tin Oxide (ITO).

To create the touch panel 10, the surface of the glass substrate 18 isflooded or covered with a substantially even layer of a resistive ITOmaterial which forms a sheet resistance. A dielectric is then applied tocover the ITO conductive material.

After applying the AC signal to the conductive ITO material 16, the nextstep is to triangulate the touch position using the current flowingthrough each corner. It is common to apply either a sine wave or asquare wave.

If an object such as a finger 14 comes in contact with the surface ofthe touch panel 10, a capacitor is formed between the ITO surface 16 andthe finger tip 14. The capacitance value is very small, typically in theorder of 50 pF. The amount of charge or current that has to be measuredgoing into each corner 12 of the panel is therefore very small. Becausethe current is so small, the system is very susceptible to straycapacitance. Thus, the accuracy of touch panels 10 is often an issue.

Accordingly, what is needed is a new method of triangulating theposition of the object on the touch panel surface that increases theaccuracy of measurements and decreases susceptibility to straycapacitance.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a touch panel thatuses a new method to determine a position of an object touching thesurface thereof.

It is another object to provide a new method of measuring current thatis less susceptible to stray capacitance.

In a first embodiment, the present invention is a touch panel having asubstantially even coating of a conductive material on a non-conductivesubstrate and then covering the conductive material with a dielectricmaterial, wherein a novel current measuring circuit reduces the effectof stray capacitance on the accuracy of a current measurement so thatthe relative position of an object on the touch panel can be determinedusing simple ratio equations.

These and other objects, features, advantages and alternative aspects ofthe present invention will become apparent to those skilled in the artfrom a consideration of the following detailed description taken incombination with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a touch panel, as found in the priorart.

FIG. 2 is a perspective view of a touch panel 10 that is made inaccordance with principles of the prior art.

FIG. 3 is a perspective view of a touch panel 10 that is made inaccordance with the principles of the present invention.

FIG. 4 is a circuit diagram showing how a sensitive current measuringcircuit comprised of a capacitor and a current measuring sensor isapplied to the touch panel.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention. It is to be understood that thefollowing description is only exemplary of the principles of the presentinvention, and should not be viewed as narrowing the claims whichfollow.

FIG. 2 shows the surface of a touch panel. 10 as found in the prior art.The lines 20 are indicative of the voltage gradient that is producedacross the surface of the touch panel when a voltage is applied at twocorners of the surface. For example, the voltage is applied at corners22 and 24 resulting in the constant voltage gradient shown. There issignificant distortion of the voltage-gradient lines 20 which is commonto many touch panels 10.

FIG. 3 is a perspective view of a touch panel 10 that is made inaccordance with the principles of the present invention. A new and novelapproach to determining the position of an object on the touch panel isto charge a large capacitor and then apply this “flying capacitor” tothe touch panel 10. In the flying capacitor method of the presentinvention, this method measures the instantaneous and total currentinduced in a contact on a surface of the touch panel 10 when a constantvoltage gradient is produced across the surface in a single axis.

Linearity of a voltage gradient can improve accuracy of the touch panel.Therefore, in a first step, it is desirable but not essential that alower resistance material be added around the edges of the touch panel10 on the surface. The voltage gradient lines 20 become closer and morelinear from a top edge 26 to a bottom edge 28.

FIG. 4 is a circuit diagram showing how a sensitive current measuringcircuit comprised of a capacitor and a current measuring sensor isapplied to the touch panel 10 in a first embodiment of the presentinvention. Any charge that is taken from the touch panel 10 is measuredwith the current measuring circuit.

In this embodiment, four measurements X1, X2, X3 and X4 must be taken inorder to determine the location of a pointing object 50 (locatedarbitrarily on the touch panel 10) on the surface of the touch panel 10.Therefore, the first step is to electrically couple a positive node ofthe flying capacitor 30 to a first side 40 of the touch panel 10 whilethe negative node is electrically coupled to an opposite second side 42of the touch panel along with a sensor or current measuring circuit 44.The current measuring circuit 44 can be an ammeter.

The voltage gradient is formed across the surface of the touch panel 10from the first side 40 to the second side 42, and to the sensor circuit44. A finger or other pointing object 50 touching the surface of thetouch panel 10 at any given point will cause a drain on the current thatis being measured by the sensor circuit 44. The drain in current to thesensor circuit 44 is a function of the distance of the finger from thefirst and second sides 40, 42 of the touch panel 10. The firstmeasurement X1 is thus the current leaving the touch panel 10 at thesecond side 44.

Assume that the first side 40 is arbitrarily left side of the touchpanel 10 as shown in FIG. 4. The second side 42 would thereforecorrespond to the right side of the touch panel 10 the first and secondsides 40, 42 are arbitrarily selected and can be switched with no changein the method of the present invention.

The second current measurement X2 is taken by switching the positive andnegative nodes of the flying capacitor 30 between the first and secondsides 40, 42 of the touch panel 10. The current measuring circuit 44 isalso moved when the circuit is reversed to take current measurement X2.

A position of the pointing object 50 can be determined as a ratio ofcurrent measurements X1 and X2. The position of the pointing object 50is a value that is easily assigned to be between zero and one, and isdetermined using equation 1:

X=X1/(X1+X2)

Two similar measurements are taken using the top 26 and bottom 28 orthird and fourth sides of the touch panel 10. The positive node of theflying capacitor 30 can be coupled to the top edge 26 or the bottom edge28 first. The decision regarding which edge to connect to the positivenode first is arbitrary. The result is current measurements Y1 and Y2. AY position ratio is then obtained using equation 2:

Y+Y1/(Y1+Y2)

The strength of the present invention as described above is that theflying capacitor 30 is used to create the high current required toproduce the constant voltage gradient on the surface of the touch panel10 and thus enable direct measurement of the current leaving the surfacethough contacts on the surface. The current induced in the lowresistance material is much larger than the current induced in thepointing object on the surface. Having a large current to measureincreases the accuracy of the system and reduces the effect that straycapacitances can have on the measurements.

It should be understood that the charge on the flying capacitor 30 israpidly being refreshed in order to maintain the voltage gradient acrossthe touch panel 10. The process of disconnecting the flying capacitor 30from the touch panel 10, refreshing the charge, and then reconnectingthe flying capacitor to the touch panel 10 is well known to thoseskilled in the art and is not an aspect of the present invention.

It is also possible to determine a Z position of the pointing objectrelative to the surface of the touch panel 10. The Z location of thepointing object is determined using equation 3:

Z=(X1+X2+Y1+Y2)/4

The advantage of the embodiment of the present invention described aboveis that a voltage gradient is formed across the touch panel 10 using arelatively crude yet simple current measuring circuit 44. Nevertheless,a measurement of the current going to the pointing object is veryprecisely measured because there is no other path for the current tofollow other than between the positive and negative nodes of the flyingcapacitor 30 and the pointing object 50.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention. The appended claims are intended tocover such modifications and arrangements.

1-13. (canceled)
 14. A method for determining the position of a pointingobject on a first axis of a surface capacitance touch panel, said methodcomprising: 1) providing a touch panel comprised of an insulatingsubstrate, a resistive material disposed on the substrate, and adielectric disposed on the resistive material; 2) connecting a chargedcapacitor to opposite first and second edges of the resistive materialand measuring current X1 through a path from the first edge to anelectrostatic ground using a current measurement circuit; 3) switchingconnections of the charged capacitor between the first and second edgesof the resistive material and measuring current X2 through a path fromthe second edge to the electrostatic ground using the currentmeasurement circuit; and 4) determining a position of the pointingobject on the first axis between the first and second edges on a surfaceof the resistive material as a ratio of measured currents X1 and X2. 15.The method as defined in claim 14 wherein the method further comprisesdetermining the position of the pointing object in a second axis that isbetween different opposite edges of the touch panel.
 16. The method asdefined in claim 15 wherein the method further comprises: 1) connectinga charged capacitor to opposite third and fourth edges of the resistivematerial and measuring current Y1 through a path from the third edge tothe electrostatic ground using the current measurement circuit; 2)switching connections of the charged capacitor between the third andfourth edges of the resistive material and measuring current Y2 througha path from the fourth edge to the electrostatic ground using thecurrent measurement circuit; and 3) determining a position of thepointing object on the second axis that is between the third and fourthedges on a surface of the resistive material as a ratio of measuredcurrents Y1 and Y2.
 17. The method as defined in claim 16 wherein themethod further comprises increasing accuracy of the method for measuringthe position of a pointing object on the touch panel by only allowingthe current to drain to the pointing object and to the currentmeasurement circuit.
 18. The method as defined in claim 16 wherein themethod further comprises determining a position of the pointing objectin the first axis using the equation X=X1/(X1+X2).
 19. The method asdefined in claim 18 wherein the method further comprises determining aposition of the pointing object in the second axis using the equationY=Y1/(Y1+Y2).
 20. The method as defined in claim 14 wherein the methodfurther comprises connecting a positive node of the capacitor to thefirst edge, and a negative node of the capacitor to the second edge inorder to measure current X1.
 21. The method as defined in claim 14wherein the method further comprises connecting a positive no of thecapacitor to the second edge, and a negative node of the capacitor tothe first edge in order to measure current X2.
 22. The method as definedin claim 15 wherein the method further comprises connecting a positivenode of the capacitor to the third edge, and a negative node of theflying capacitor to the fourth edge in order to measure current Y1. 23.The method as defined in claim 15 wherein the method further comprisesconnecting a positive node of the capacitor to the fourth edge, and anegative node of the capacitor to the third edge in order to measurecurrent Y2.
 24. The method as defined in claim 14 wherein the methodfurther comprises providing a known voltage on the capacitor in order todetermine how much current is being drained from the resistive materialby the presence of the pointing object.
 25. The method as defined inclaim 24 wherein the method further comprises refreshing the capacitorto the known voltage in order to maintain the voltage gradient acrossthe resistive material.
 26. The method as defined in claim 14 whereinthe method further comprises creating a linear voltage gradient acrossthe resistive material.
 27. The method as defined in claim 14 whereinthe method further comprises using an ammeter as the current measuringcircuit.
 28. The method as defined in claim 15 wherein the methodfurther comprises determining the position of the pointing object in anyaxis that is defined as being disposed between any two opposite edges ofthe touch panel.
 29. A method for determining the position of a pointingobject on a first axis of a surface capacitance touch panel, said methodcomprising: 1) providing a touch panel comprised of an insulatingsubstrate, a resistive material disposed on the substrate, and adielectric disposed on the resistive material; 2) providing a voltagegradient from a flying capacitor across a resistive material from afirst edge to a second edge, and measuring a first current through apath from the first edge to an electrostatic ground using a currentmeasurement circuit; 3) switching connections of the flying capacitorbetween the first and second edges of the resistive material andmeasuring a second current through a path from the second edge to anelectrostatic ground using the current measurement circuit; and 4)determining a position of the pointing object on the first axis betweenthe first and second edges on a surface of the resistive material as aratio of measured first and second currents.
 30. A surface capacitancetouch panel that can determine the position of pointing object on afirst axis thereof, said surface capacitance touch panel comprised of:an insulating substrate; a resistive material disposed on the substrate;a dielectric disposed on the resistive material; a flying capacitor forthat is switchably coupled to any opposite edges of the resistivematerial to thereby selective provide a voltage gradient across theresistive material from one edge to an opposite edge; a currentmeasuring circuit that is coupled to one end of the flying capacitor andto electrostatic ground, to thereby measure current through a path froman edge to electrostatic ground; ands means for determining a positionof the pointing object on the first axis between the opposite edges on asurface of the resistive material as a ratio of measured currents.